1. Field of the Invention
The present invention relates to a method for horizontal or vertical continuous casting of an aluminum-lithium based alloy, and, more particularly to an improved contacting method of the molten metal with gas. The contacting method mentioned above particularly relates to the gas-pressure application method in hot-top casting.
2. Description of Related Arts
The aluminum-lithium based alloy exhibits a low density, high strength and elasticity, and excellent fracture toughness. This alloy is therefore presently used mainly for construct-ional materials of air crafts. Active development has also been made which discovered a composition of the aluminum-lithium alloy having high strength per weight.
Usually, an aluminum-lithium based alloy is continuously cast to form a sheet, slab or billet, and is then rolled to form a rolled sheet or is extruded to form extruded profiles. The rolled sheet or extruded profiles may further be subjected to plastic working.
The continuous casting is carried out mainly by the direct chill method. Since the qualities of the continuously cast ingot have been recently enhanced, the rolling and extruding steps can be omitted. In order to reduce the cost, it is desirable to produce an ingot having a small cross section by continuous casting, and to use such ingot directly as a product, or subject it to rolling.
The aluminum-lithium alloy is active and is hence usually molten and then cast in an inert atmosphere. Particularly, when the lithium content of the alloy is high, the molten metal of aluminum-lithium alloy is caused to react vigorously with water which is usually used as the cooling medium in the casting operation. Since the danger of explosion is involved, strict caution should be taken to avoid explosion.
Japanese Unexamined Patent Publication No. 60-250860 proposes a method to exclude the contact of the surface of molten metal with air so as to avoid explosions. According to the proposal, the space above the level of molten metal in the mold, into which the molten metal is introduced, is closed, and this space is filled with inert gas. The inert gas prevents an oxide film from forming on the active surface of the molten metal and hence leads to the production of an excellent, continuously cast ingot.
Japanese Unexamined Patent Publication No. 60-127059 proposes using, as the cooling medium in the casting operation, an organic cooling medium which contains in particular 75% or more of ethylene glycol. In the proposed method it is intended that even when the so-called break out, i.e., flowing of molten metal through the solidified shell, occurs during casting, direct contact of the molten metal with water, which would result in a strong explosion, would be prevented.
In Japanese Unexamined Patent Publication No. 60-180656 which is related to a vertical direct chill semi-continuous casting, the cooling water is continuously pumped from the pit of a casting machine so as to avoid the stagnation of water in the pit, and hence a strong explosion due to the contact of the molten metal of aluminum-lithium alloy with water.
Japanese Unexamined Patent Publication No. 62-104652 discloses a clad casting method, in which an ingot comprising an outer shell made of a lithium-free aluminum alloy, and a core made of lithium-containing aluminum alloy is continuously cast. Allegedly, the aluminum-lithium alloy is essentially prevented from contact with the cooling water in the DC (direct chill) casting operation, thereby avoiding the explosive reaction between water and lithium.
U.S. Pat. No. 4,157,728 (Japanese Examined Patent Publication No. 54-42847) discloses an improved DC casting method by means of applying gas pressure to the molten metal being cast.
Japanese Unexamined Patent Publication No. 61-71157 discloses the elimination, in the horizontal continuous casting, of the unbalanced cooling of molten metal in a mold and non-uniformity of the lubricating surface on the inner wall of a mold, thereby homogenizing the cast structure, eliminating the cast defect and break out, and hence constantly casting ingots of good quality. The method proposed in the above-mentioned publication is such that gas is introduced into a corner which is formed by a tubular mold and a protruding part of a refractory plate which is provided with an aperture for introducing the molten metal into the tubular mold. The protruding part of the refractory plate protrudes toward the inner wall of the tubular mold and forms, together with said inner wall, the above mentioned corner below the axial line of the tubular mold. The gas introduced into the corner forms a space where the gas pressure is applied. The gas pressure applied causes such a deviation of the contact position of the molten metal with the inner wall of tubular mold that it shifts horizontally toward the downstream side. Such deviation results in controlling the cooling of the molten metal at contact position mentioned above.
According to the above described method for filling the closed space with the inert gas above the level of molten metal, the contact of molten metal with air can be prevented in order to suppress the formation of oxide. However, a large amount of lubricating agent is necessary, and, even if a large amount of lubricating oil is supplied, rough cast skin is formed during a long period of operation which makes the casting operation instable.
According to the method for using the cooling agent which mainly consists of ethylene glycol and prevents explosion upon break out, the casting plant needs to be fundamentally re-adjusted and control of the operating conditions become very complicated. Cast skin does not appear to be improved by the above mentioned method.
The method for constantly draining the cooling water in the pit is undoubtedly an appropriate safety countermeasure to avoid danger in the case of break out. However, the casting plant needs to be reconstructed and the operating conditions need to be strictly controlled.
The clad casting method is complicated and the casting conditions need to be strictly controlled. In addition, the outer shell must be peeled later.
The present inventors applied the gas-pressure application method disclosed in U.S. Pat. No. 4,157,728 to aluminum-lithium alloy. They used not only air but also such inert gases as argon and nitrogen for the pressure-application. However, the molten metal stuck on the mold wall to form the sticking skin. It was therefore confirmed that the gas pressure application method is not as effective for the aluminum-lithium alloy as it is for aluminum and its lithium-free alloys.
The present inventors also carried out the horizontal continuous casting method of aluminum-lithium alloy under the application of gas pressure. The sticking of the cast skin was also drastic. In addition, open cracks were formed on the cast skin in a circumferential direction around the ingot. The open cracks resulted in not only cast defects but also operation troubles due to the flowing of molten metal through them.